1. Field of the Invention
This invention generally relates to the fabrication of integrated circuit (IC) devices, and more particularly, to a method for additionally oxidizing an oxide film using a high-density plasma oxidation process.
2. Description of the Related Art
The quality of polysilicon thin-films and the interface between silicon and silicon dioxide (Si/SiO2) layers are critical to the performance of thin-film transistors, MOS capacitors, and various ICs. The quality of the SiO2/Si interface is dependent upon the quality of the SiOx (where x is less than, or equal to 2) transition layer at the interface and the defects in the poly-Si layer. The general approach is to improve the quality of the SiOx transition layer at the Si/SiO2 interface. Defects in the poly-Si can also be passivated and the stoichiometry improved by oxidation and hydrogenation processes. These same issues also apply to semiconductor processes that form other types of oxide thin-films.
Although lower temperatures are generally desirable for any device fabrication process, they are especially critical in LCD manufacture, where large-scale devices are formed on a transparent glass, quartz, or plastic substrate. These transparent substrates can be damaged when exposed to temperatures exceeding 650 degrees C. To address this temperature issue, low-temperature Si oxidation processes have been developed. These processes use a high-density plasma source such as an inductively coupled plasma (ICP) source, and are able to form Si oxide with a quality comparable to 1200 degree C. thermal oxidation methods.
Various semiconductor devices require the deposition of SiO2, or other oxide thin-films, on structures that are both planar and non-planar. For planar surfaces there is usually no problem in depositing uniform SiO2 thin-films over large areas in the fabrication of stable and reliable devices. However, for a device with vertical steps in the structure, such as shallow-trench isolation (STI), vertical thin-film transistors (V-TFTs), graded steps, or curved surfaces, it is important to deposit SiO2 films with sufficient step-coverage to maintain film integrity, device performance, and yield. Thermal oxide has proven to be the most suitable oxide from the step-coverage point of view. However, the low growth rates and high processing temperatures exceeding 800° C. make thermal oxidation unsuitable for low-temperature devices.
Plasma-enhanced chemical vapor deposition (PECVD) processes are suitable for the low temperature processing of the SiO2 thin-films. The electrical quality and the step-coverage of the PECVD deposited oxide thin-film are strongly dependent upon the processing conditions. It is possible to improve the step-coverage of the deposited oxide by decreasing the process temperature or varying the process chemistries and plasma process variables. However, any such attempt to improve the step-coverage results in a corresponding decrease in the oxide quality.
Generally, a fixed oxide charge is a positive charge that remains, after annealing out interface trap charges, and is caused as a result of a structural defect. These fixed oxide charges occur primarily within 2 nanometers of a Si/SiO2 interface. The charge density is dependent upon oxidation and hydrogenation processes. It is known that these fixed oxide charges can be minimized through the use of high oxidation temperatures. Fixed oxide charges in a gate oxide layer can act to degrade the threshold voltage of a transistor.
Oxide trapped charges can be formed at the interface between a silicon layer and a metal or Si substrate, or can be introduced throughout the oxide layer as a result of ion implantation. Mobile ionic charges can also be formed at the silicon oxide interface as a result of ionized alkali metals, sodium, or potassium. A gate insulator with any of the above-mentioned oxide charge types can degrade the threshold voltage, breakdown voltage, and current gain of a transistor.
There are two important factors that dictate the quality of the oxide thin films: Oxygen vacancies and impurities. Oxygen vacancies and impurities result in poorer electrical performance, stability, and reliability. There are various sources for the impurities in thin oxide films, such as the substrate material, deposition method, fabrication technique and setup, precursor composition and purity, device processing methods/steps, to name a few. One common impurity in thin oxide films is carbon (C), which can be induced by the system or process. C impurities can also come from the substrate, in the case of SiC-based devices. It is important to reduce the level of C or other impurities in thin films to fabricate reliable electronic devices.
The issue of incomplete oxidation can be addressed by exposing the films to oxygen atmosphere, while the carbon type impurities can be effectively removed by conversion of the C to CO or CO2, which diffuses out of the film. One common approach to improve oxidation and minimize the C type impurities in oxide thin films is a post-deposition treatment in an oxygen atmosphere. However, the thermal oxidation processes have the major limitation of a high thermal budget (high temperature/long oxidation time), which is not suitable for low-temperature devices. Additionally, the thermal oxidation process has a low oxidation efficiency due to the molecular state of the oxygen species, and often results in undesirable interactions, such as diffusion, among various layers in the devices. Rapid thermal oxidation processes also show poor oxidation efficiency, especially at film thicknesses greater than 100 Å, and the thermal budget is not suitable for low-temperature (<600° C.) devices integrated on glass, plastic, polymer, or other low temperature substrates.
Plasma oxidation processes have a lower thermal budget and the higher efficiency than thermal processes. However, the oxide formed from a conventional capacitively-coupled plasma (CCP) generated plasma may create reliability issues due to the high bombardment energy of the impinging ionic species. It is important to control or minimize any plasma-induced bulk or interface damage. However, it is not possible to control the ion energy using radio frequency (RF) of CCP generated plasma. Additionally, the low plasma density associated with these types of sources (˜108-109 cm−3) leads to inefficient oxidation and impurity reduction at low thermal budgets, which limits their usefulness in the fabrication of low-temperature electronic devices.
Other approaches such as radical oxidation, photo oxidation, and ozone oxidation, have been taken to improve the oxidation efficiency, by the creation of an active oxygen species. However, the process complexity, plasma density, nature and life-time of active species, low efficiency, high thermal budgets, and large area processing limit the applicability of these approaches to low-temperature and high-throughput device fabrication.